This invention relates to an exposure apparatus used in fabrication of semiconductor devices, and more particularly to a reduction-projective type excimer exposure apparatus which is capable of realizing an ultrafine processing by the technique of photolithography in fabrication of semiconductor devices.
Presently, there are already commercial products of reduction-projective type exposure apparatus (stepper) using a superhigh pressure mercury lamp as the light source for use in fine processing of semiconductor devices, especially LSI and VLSI. In the existing steppers, however, since g-line (435 nm) or i-line (365 nm) of superhigh pressure mercury lamp is used as the light for exposure, the resolution was limited at about 1.2 .mu.m in g-line and about 0.8 .mu.m in i-line. At such wavelengths, it is nearly impossible to obtain a resolution of 0.5 .mu.m that is said to be necessary for fabrication of 4-Mbit DRAM or 16-Mbit DRAM henceforth.
Recently, interests are inclined to the development of exposure apparatus for forming sub-micron pattern using excimer light source, such as XeCl (308 nm), KrF (249 nm) and ArF (193 nm) having shorter wavelengths than g-line or i-line.
However, the reduction-projective exposure using excimer laser involves the following two problems.
(1) The achromatic problem by the half-value width of waveform distribution of the light oscillated by excimer laser beam and the reduction-projective lens (relation between speckle and resolution).
(2) The problem of selection of alignment wavelength.
The first problem may be solved by developing an optical system which can adjust the vertical mode while disturbing the lateral mode of the excimer laser beam, but in the second problem of selection of alignment light, it is preferable to employ a method of directly aligning by the reticle, mask and mark of wafer through a reduction-projective lens by using the same light as the exposure wavelength in order to enhance the alignment precision (which is called the through-the-lens method or TTL method here in after), and it is advantageous in that the lens designing is easy. Never the less, when aligned by using the light of same wavelength as the exposure wavelength, the resist in the alignment key part is exposed during alignment, and the alignment key is broken in the subsequent process. Generally, therefore, in the alignment by TTL method, it was common to use alignment light and exposure wavelength in different regions, for example, conventionally, to use d-line (577 nm) or e-line (546 nm) or Ar laser (515 nm) as alignment light in contrast to the exposure wavelength of g line or i-line of superhigh pressure mercury lamp.
However, when excimer laser light such as KrF (248 nm) is used in the exposure wavelength, lens materials usable in the reduction lens system are limited (practically SiO.sub.2 or CaF.sub.2), and when the alignment light too different in the wavelength from the exposure wavelength is used, the chromatic aberration cannot be corrected completely, and the resolution is much deteriorated, there by making it impossible to align. That is, it has been discovered that the deviation of d-line (577 nm), e-line (546 nm) of superhigh pressure mercury lamp or Ar laser (515 nm) from the excimer light of exposure wavelength 248 nm is too large to be used in the alignment.
On the other hand, to use the light having the same wavelength as the KrF excimer light source as the alignment light has too many disadvantages as mentioned above.